On the same day it was revealed that a Shanghai high school got the go-ahead from the municipal education bureau to run two all-boys' classes, the authorities reported that the ratio of newborn boys to every 100 girls born on the mainland last year was 117.78. At that rate, there would be no shortage of boys to fill the classes while maintaining even numbers of boys and girls in co-educational classes. But that is not why Shanghai No 8 Senior High School wanted to introduce all-male classes. The headmaster sees a male-oriented environment as the remedy to inferior academic performance and leadership qualities among boys who are taught alongside girls. Similar perceptions have long been the subject of debate in secular societies around the world, and some mainland education academics were quick to debunk them. The new all-boys classes are therefore a pilot project to be watched with interest. The newborn gender imbalance is increasingly a rural phenomenon, attributed to a traditional preference for boys over girls, and the one-child policy and its limited dispensations. Demographer He Yafu, based in Guangdong, says the ratio of boys to girls born in the countryside can be as high as 150. One social benefit of mass urban migration has been a weakening of that preference. Even though the ratio of male to female births has fallen from 119.45 to 117.78 over three years, it remains among the world's highest, magnified by the mainland's huge population. China is still heading for a surplus of 24 million men of marriageable age by 2020. That cannot be good for social stability. The authorities must redouble their efforts to bring it much closer to the natural ratio of 103 to 107. They can take heart from evidence that a crackdown on abuse of ultrasonic screening to determine an unborn child's gender, and sex-selective abortion, is bearing results, along with campaigns encouraging care for girls. While urbanisation will continue to correct the trend, they should focus their efforts on more populous regions of the countryside.